Kinetic determination of acarbose and miglitol in bulk and pharmaceutical formulations using alkaline potassium permanganate.

A simple and sensitive kinetic spectrophotometric method was established for the determination of acarbose and miglitol in bulk and in their pharmaceutical preparations using alkaline potassium permanganate as an oxidizing agent. The method involves determination of acarbose and miglitol by kinetic studies of their oxidation at room temperature for a fixed time of 15 minutes for acarbose and 25 minutes for miglitol. The absorbance of the colored manganate ion was measured at 610 nm. Alternatively, the kinetic decrease in the absorbance of permanganate upon addition of the studied drugs at 525 nm was also used. The absorbance concentration plot was rectilinear over the concentration range of 4-20 and 1-10 μg/ml for acarbose and miglitol, respectively. The detection limits were 0.189 and 0.089 μg/ml at 610 nm and 0.081 and 0.179 μg/ml at 525 nm for acarbose and miglitol respectively. The method was successfully applied for the determination of these drugs in their dosage forms. The results obtained were in good agreement with those obtained with the reference methods.

To the best of our knowledge, no spectrophotometric methods have been reported for the analysis of acarbose and miglitol up till now. The results obtained were promising.
The aim of the present work was to study the reaction between the studied drugs with potassium permanganate in alkaline medium kinetically in an attempt to evaluate them in their dosage forms. The proposed method was simple and did not need sophisticated instruments or special skill, sensitive, rapid and readily adaptable to both the bulk drug and dosage forms.

EXPErIMENtAL reagents
All chemicals used were of analytical reagent grade and the solvents were of spectroscopic grade.

Materials
The different pharmaceutical preparations were purchased from the commercial source in the local market.

stock solutions
Stock solutions of acarbose and miglitol were prepared by dissolving 100.0 mg of the studied drugs in 100 ml distilled water. Other concentrations were prepared by further dilution with distilled water. These solutions alsowere found stable for at least three days without alteration when kept in the refrigerator.

GENErAL PrOcEDUrEs construction of the kinetic calibration graphs
Aliquot solutions containing 40-200 µg of standard acarbose and 10-100 µg of miglitol solutions were transferred into a series of 10-ml volumetric flasks. 1 ml of 0.5 M NaOH, followed by 1 or 2.5 ml of 1 × 10 -2 M potassium permanaganate for acarbose and miglitol, respectively at 610 nm or 0.5 ml of 7.6 × 10 -3 M for both drugs at 525 nm were added. The mixture was shaken well and completed to volume with distilled water. The increase at 610 nm or the decrease at 525 nm in the absorbance was scanned during 15 and 25 min. for acarbose and miglitol, respectively at ambient temperature (25°C) against an appropriate blank, prepared simultaneously. The reaction order was obtained by plotting log reaction rate (ΔA/Δt) over the specified time period versus log concentration of the drug. The calibration graphs and the regression equations were obtained by plotting the absorbance (A) or the difference in absorbance (ΔA) at the specified time versus concentration of the drug in µg/ml.

Procedure for the Determination of the studied compounds in Dosage Forms
An accurately weighed quantity of the mixed contents of 10 powdered tablets equivalent to 100.0 mg of the drug was transferred into a 100 ml volumetric flask. The content of the flask was completed to 100 ml with distilled water then sonicated for 15 minutes and filtered. An aliquot of the cited solutions was taken and the above procedure was applied. The nominal content was calculated either from a previously plotted calibration graph or using the regression equation.

rEsULts AND DIscUssION
Oxidation of the studied drugs ( Fig. 1) with KMnO 4 was carried out in presence of NaOH. Trials were made to determine the drug through oxidation with KMnO 4 in neutral and acidic media, but no apparent reaction products were observed. Potassium permanganate in alkaline medium oxidized the studied drugs and yielded the green color of manganate radical, which absorbs maximally at 610 nm (Fig. 2). The intensity of the color was increased with time, and so, a kinetic method was developed for the determination of these drugs at 610 nm. An alternative kinetic method for the determination of both drugs based upon measuring the decrease in the absorbance of KMnO 4 at 525 nm was also developed.
Other oxidants were tested to determine the studied drugs, such as 10% H 2 O 2 , potassium persulphate in alkaline medium and potassium periodate in strong acid medium but all failed to give satisfactory results In case of H 2 O 2 and persulphate, complete decomposition of the drug was observed, as revealed by the absence of any absorbing species. In case of periodate, oxidation of the drug resulted in hypsochromic shift and hypochromic effect, with max-imum absorbance at 236 nm and this was in agreement with the reported results of oxidation of amino-alcohol compounds (26).

study of Experimental Parameters
The different experimental parameters affecting the formation of the oxidation product were studied. Variables were optimized by changing each in turn, while, keeping all others constant.
Effect of KMnO 4 concentration. The influence of KMnO 4 concentration on the absorbance of the reaction product was studied using different volumes (0.2-3.5 ml) of 1 × 10 -2 M KMnO 4 . The reaction rate and hence maximum absorbance increased with increasing KMnO 4 concentration at 610 nm. It was found that at least 1 or 2.5 ml of 1 × 10 -2 M KMnO 4 was adequate for the maximum absorbance of acarbose and miglitol, respectively as shown in Fig. 3, 0.5 ml of 7.6 × 10 -3 M was sufficient for measuring the decrease in the absorbance at 525 nm for both drugs.
Effect of sodium hydroxide concentration. The influence of the concentration of NaOH on the absorbance of the reaction product was studied using different volumes (0.2-1.4 ml) of 0.5 M NaOH. It was found that increasing the volume of 0.5 M NaOH would increase A or ΔA of the reaction up to 1 ml for both drugs at 610 nm or 525 nm after that NaOH has no effect on the absorbance as shown in Figures 4 & 5. Effect of temperature. The effect of temperature on the reaction rate was studied, it was found that, permanganate was reduced to manganate radical at room temperature (25°C) while at higher temperatures, manganese   dioxide was produced. Therefore, room temperature was selected as the optimum temperature.
Effect of time. The effect of time on the reaction between KMnO 4 and the studied drugs was studied. The absorbance of the reaction mixture was increased with time and never reach maximum in a reasonable time. Quantification was therefore made at fixed times of 15 min. for acarbose and 25 min. for miglitol (Figs. 6-9).

Evaluation of the Kinetic Parameters
As mentioned above, the reaction between KMnO 4 and the studied drugs never reach completion and a decision was made to apply a kinetic method for their determination. Consequently, the order of the reaction and reaction rate constants were determined at 610 and 525 nm.
The rate of the reaction was found to be dependent on acarbose and miglitol concentrations. The rates were followed at room temperature with various concentrations in the range of 4-20 μg/ml for acarbose and 1-10 μg/ml for miglitol keeping KMnO 4 and NaOH concentrations constant at the recommended levels mentioned above. The reaction rate obeys the following equation: (a) where K` is the pseudo-order rate constant and n is the order of the reaction.
The rate of the reaction may be estimated by the variable time method measurement (27), where A is the absorbance and t is the time in seconds. Taking logarithms     Table 1. These results indicate that the reaction is pseudo first order reaction in the drug concentration.

selection of the best kinetic method
Several kinetic techniques were adopted for the selection of the best method. Rate constant, fixed absorbance and fixed time methods (28,29) were tried and the most suitable analytical method was selected taking into account the applicability, the sensitivity, i.e. the slope of the calibration graph and the correlation coefficient (r).
Rate constant method. Graphs of log absorbance versus time for acarbose and miglitol concentration in the range of 6.20 × 10 -6 -3.10 × 10 -5 M and 4.83 × 10 -6 -4.83 × 10 -5 M, respectively were plotted and all appeared to be rectilinear. Pseudo-first order rate constants (K`) corresponding to different drug concentrations (C) were calculated from the slopes multiplied by -2.303 and are presented in Table 2.
Fixed time method. At a preselected fixed time, which was accurately determined, the absorbance was measured. Calibration graphs of the absorbance versus initial concentrations of acarbose and miglitol at fixed times of 15 and 25 min., respectively were established with the regression equations and correlation coefficients assembled in Table 4.
It is clear that the slope increases with time and the most acceptable values of the correlation coefficient (r) was chosen as the most suitable time interval for the measurement.  As a conclusion, the fixed time method was chosen for quantification because it gave the best correlation coefficient in a reasonable time.

Quantification
After optimizing the reaction conditions, the fixed time method was applied to the kinetic determination of 4-20 µg/ml of acarbose and 1-10 µg/ml of miglitol in raw materials.
Statistical evaluation of the regression line gave the values of S y/x , S a , S b which are indicated with the detection limits, quantification limits, % RSD and % Er. in Table 5.
These small values point out to the high precision of the proposed methods.
Statistical analysis of the results obtained by both the proposed and reference methods (30,31) revealed no sig- The proposed methods were successfully applied for the determination of the studied drugs in their different dosage forms. The results obtained were in a good agree-   ment with the reference methods (30,31), as shown in Table 7.

Mechanism of the reaction
The data used in the optimization of KMnO 4 concentration and the data of the calibration graphs were used to calculate the stoichiometry of the reaction adopting the limiting logarithmic method (33).
The ratio of the reaction between acarbose or miglitol and KMnO 4 in alkaline medium was calculated by dividing the slope of KMnO 4 curve over the slope of the drug curve (Figs. 10 & 11). It was found that, the ratios were 1.012: 0.951 for acarbose and 0.943: 0.990 for miglitol pointing out to a ratio of 1:1 (KMnO 4 to drug).
Based on the obtained molar reactivity, the reaction pathway is proposed to proceed as in Figure 12.

cONcLUsION
The proposed methods are simple, accurate, precise, sensitive, rapid, low cost and selective compared to the reference methods (30,31).
Furthermore, the proposed methods don't require elaboration of procedures, which are usually associated with chromatographic methods. The proposed methods were applied successfully for determination of the studied compounds in raw material as well as in different dosage forms. The only limitation for this method, if used in other pharmaceutical preparations containing antioxidant which will cause interference and this can be solved by using suitable solvent extraction.   From the above study some specific advantages in the application of kinetic methods can be expected (35): • Selectivity due to the measurement of the evolution of the absorbance with the time of reaction instead of the measure of a concerete absorbance value. • Possibility of no interference of other absorbent active compounds present in the commercial product, if they are exhibiting stability in the chemical reaction conditions established for the proposed kinetic method. • Possibility of no interference of the colored and /or turbidity back ground of the sample.